Heteropolymolybdate catalysts and method for oxydehydrogenation

ABSTRACT

Catalysts containing molybdenum, vanadium, phosphorus, and oxygen, optionally together with one or more metal ions, as their active constituents, are preferably disposed on an inert carrier for the oxidative dehydrogenation of isobutyric acid or its lower esters to methacrylic acid or its lower esters.

This application is a continuation-in-part of application Ser. No.07/390,487, filed Aug. 1, 1989 and now abandoned, which in turn is acontinuation of application Ser. No. 07/078,223 filed July 27, 1987 andnow abandoned.

The present invention relates to mixtures of heteropolyacid compounds,particularly to mixtures of phosphomolybdic acid and vanadium-containingderivatives of phosphomolybdic acid and their salts, and to methods forthe oxydehydrogenation of isobutyric acid or its esters to methacrylicacid or its esters using such mixtures as catalysts.

THE PRIOR ART

12-Molybdophosphoric acid, H₃ PMo₁₂ O₄₀ .sup.. xH₂ O, also known asphosphomolybdic acid, can be prepared, for example, by reactingmolybdenum trioxide in heated dilute phosphoric acid, in other words,hydrothermally. [Tsigdinos, Ind. Eng. Chem., Prod. Res. Develop. 13, 267(1974)].

This phosphomolybdic acid can be used as a heteropolyacid catalyst inthe oxydehydrogenation of isobutyric acid to methacrylic acid, whichproceeds by heterogeneous catalysis. However, the selectivity of thiscatalytically active substance for methacrylic acid has anunsatisfactory value of only 40 percent. [Otake et al., Studies SurfaceScience and Catalysis 7, 780-791 (1981)]. In contrast thereto, aheteropolyacid catalyst with H₅ PMo₁₀ V₂ O₄₀, a vanadium derivative ofphosphomolybdic acid with a selectivity for methacrylic acid of 73percent, is considerably more selective and has practically the sameactivity, according to the same authors. Vanadium derivatives ofphosphomclybdic acid such as H₅ PMo₁₀ V₂ O₄₀ or H₄ PMo₁₁ VO₄₀ can beprepared inter alia hydrothermally, as described in German PatentPublication 27 22 375, (U.S. Pat. No. 4,146,574) by heatingstoichiometric amounts of MoO₃ and V₂ O₅ in water containing H₃ PO₄.

As the applicant's own tests have shown, oxydehydrogenation catalystscontaining the heteropolyacid H₄ PMo₁₁ VO₄₀ as the catalytically activesubstance, which catalysts have a selectivity for methacrylic acid ofabout 60 percent, are significantly less selective thanoxydehydrogenation catalysts containing H₅ PMo₁₀ V₂ O₄₀. While H₃ PMo₁₂O₄₀ is quite unsuitable for use as a catalyst in a commercial processbecause of its low selectivity, H₄ PMo₁₁ VO₄₀, used as a catalyst, andespecially the relatively selective H₅ PMo₁₀ V₂ O₄₀ or H₆ PMo₉ V₃ O₄₀catalysts, lack the long term characteristics required from thetechnical and economic points of view.

Catalysts prepared from metallic salts of heteropolyacids, for exampleH₇.6 Cu₀.2 PMo₁₀ VO₃₉ or its anhydride, Cu₀.2 PMo₁₀ VO₃₅.2, which areknown from German Patent Publication 32 48 600 for theoxydehydrogenation of isobutyric acid or its esters and which areprepared hydrothermally in the presence of a copper compound and havehigh selectivities for methacrylic acid of up to about 83 percent in theoxydehydrogenation of isobutyric acid, also exhibit an unsatisfactorylong term behavior.

THE OBJECT AND THE INVENTION

Thus, there has been a need for developing heteropolyacid derivativescomprising phosphomolybdic acid which, when used as catalysts for theoxydehydrogenation of isobutyric acid or its esters to methacrylic acidor its esters, exhibit not only good activity and selectivity but alsoexhibit long term behavior which is technically satisfactory.

According to the present invention, it has been found that compositionsof various heteropolyacids comprising phosphomolybdic acid which have anatomic ratio of Mo:V wherein Mo is greater than 9 and less than 12 and Vis 0.1 or greater but less than 3 surprisingly have long term stabilitywhen used as oxydehydrogenation catalysts. The heteropolyacidderivatives used in accordance with the invention are compositions ofvarious heteropolyacids comprising phosphomolybdic acid, such as H₃PMo₁₂ O₄₀ and/or H_(3+x) PMo_(12-x) V_(x) O₄₀, wherein x is 1, 2, or 3,or salt derivatives thereof, and they may be mixtures of the variousstarting heteropolyacids or their salts as well as mixtures containingfurther compounds formed during preparation.

The invention thus relates to catalysts containing molybdenum, vanadium,phosphorus, and oxygen, and optionally one or more metal ions, as theactive constituents, which catalysts are preferably disposed on an inertcarrier and are useful for the oxidative dehydrogenation of isobutyricacid or its lower esters to form methacrylic acid or its esters. Thecatalytically active constituent of the catalyst is, or is formed from,a heteropolymolybdate mixture of phosphomolybdic acid, H₃ PMo₁₂ O₄₀, orits salts and/or its vanadium derivatives, H_(3+x) PMo_(12-x) V_(x) O₄₀,wherein x is 1, 2, or 3, and/or their salts.

The catalyst corresponds to a heteropolymolybdate mixture of the formulaH_(a) M_(b) P_(c) Mo_(d) V_(e) O_(f), wherein M is a cation of one ormore of the metallic elements Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Zn,Cd, Al, Ce, Ti, Zr, Sn, Sb, Pb, Bi, Cr, Mn, Fe, Co, Ni, Cu, Ag, or Asand a is greater than 0.1 but less than 6, b has a value from 0 to 3, chas a value from 0.5 to 1.5, d has a value greater than 9 but less than12, e has a value from 0.1 to 3, and f has a value determined by thevalences and amounts of the other elements represented in the formula.The heteropolymolybdate mixtures preferably contain Cu, Cs, Rb, K, Ti,Ce, Mn, Fe, Bi, Cr, or As as component M.

Surprisingly, the catalysts prepared from the compositions in accordancewith the invention have a life longer by a factor of from 5 to about 100than prior art H₅ PMo₁₀ V₂ O₄₀ or Cu₀.2 PMo₁₀ VO₃₅.2 heteropolyacidcatalysts that are catalytically active and selective inoxydehydrogenation. Hence the new catalysts have a correspondinglyhigher productivity, expressed, for example, as kg of methacrylic acidper gram of catalytically effective heteropoly acids, and thus offer adecisive advantage over the prior art oxydehydrogenation catalysts.

The novel catalysts for the oxydehydrogenation of isobutyric acid or itsesters are heteropolyacid and/or heteropolyacid salt mixtures comprisingphosphomolybdic acid which are characterized in that the mixtures havean atomic ratio of Mo:V wherein Mo is greater than 9 but less than 12and V is 0.1 or greater but less than 3, with an Mo:P atomic ratiowherein Mo is greater than 9 but less than 12 and P is from 0.5 to 1.5,preferably 1.

PRACTICE OF THE INVENTION

From the Mo-V-P atomic ratios indicated, which ratios are not limited tointegers, it is apparent that the novel compositions, for examplemixtures of various known heteropolyacids or their salt derivatives,such as of H₃ PMO₁₂ O₄₀ with H₄ PMo₁₁ VO₄₀ and/or with H₅ PMo₁₀ V₂ O₄₀and/or with H₂.4 Cu₀.8 PMo₁₁ VO₄₀, or also of H₄ PMo₁₁ VO₄₀ with H₅PMo₁₀ V₂ O₄₀, may have different molar ratios and, as has been found,can be prepared by intensive mixing of such constituents. When used ascatalysts in the oxydehydrogenation of isobutyric acid or its esters,mixtures of the heteropolyacids, such as a 1:1 molar mixture of H₃ PMo₁₂O₄₀ and H₄ PMo₁₁ VO₄₀ which then has the overall composition H₃.5PMo₁₁.5 V₀.5 H₄₀, exhibit not only good activity but also significantlyimproved long term behavior.

However, it has further been found that the selectivity of the inventivemixed heteropolyacid catalysts is improved still more, and considerablyso, if they contain metal ions. Heteropolyacid mixtures containing metalions can be prepared, as indicated above, by admixing an appropriatemetallic salt of a heteropolyacid, or by reacting at least a portion ofthe heteropolyacids present in the heteropolyacid mixture with furthermetal compounds to give their metallic salts. In this way, catalysts areobtained which not only have good activity and considerably improvedlong term properties, but also substantially better selectivity formethacrylic acid. A content of copper ion in the heteropoly acidmixtures has proved particularly advantageous. Such compositions areobtained, for example, by reacting heteropoly acid mixtures according tothe invention with copper hydroxide, for example. With a composition ofthe invention such as H₃.1 Cu₀.2 PMo₁₁.5 V₀.5 O₄₀ used as a catalyst,selectivities for methacrylic acid of about 70 percent have beenobtained, which represents an increase of more than 20 percent over thatof a copper free catalyst. Substantial stabilization of this selectivityis secured by adding further metal ions, for example in a Cu/Cs metalion combination.

Thus, the catalysts of the present invention are (A) a mixture of atleast two members selected from the group consisting of

(i) H_(3+x) PMo_(12-x) V_(x) O₄₀, wherein x is 1, 2, or 3 and

(ii) metals salts thereof, the molar ratio of any member or combinationof members to any other member in such a mixture being from 0.99:0.001to 0.01:0.99, or consisting essentially of

(B) a mixture of at lest one member selected from the group consistingof (A)(i) and (A)(ii) and at least one member selected from the groupconsisting of

(iii) H₃ PMo₁₂ O₄₀ and

(iv) metal salts thereof,

the molar ratio of said member or members (A)(i) and (A)(ii) to saidmember or members (B)(iii) and (B)(iv) being from 0.9:01 to 0.1:0.9.

The composition of the inventive heteropoly acid mixture containingmetal cations can be represented by the formula

    H.sub.a M.sub.b P.sub.c Mo.sub.d V.sub.e O.sub.f,

wherein M signifies one or more of the elements Li, Na, K, Rb, Cs, Be,Mg, Ca, Sr, Ba, Zn, Cd, Al, Ce, Ti, Zr, Sn, Sb, Pb, Bi, Cr, Mn, Fe, Co,Ni, Cu, Ag, or As and wherein a, b, c, d, e, and f have the values givenearlier.

The preparation of the heteropolyacid metallic salts in admixture withfree heteropolyacid is advantageously carried out by adding metalcompounds such as metal hydroxides, metal carbonates, metal nitrates, ormetal phosphates to mixtures of heteropolyacids, which latter can bemade for example, as a paste with water to improve mixing. For betterconversion to the heteropolyacid metallic salt, the mixture may beheated for a short time to about 100° C.

The preparation of the heteropolymolybdate mixture in the presence of asolid carrier is particularly preferred for preparation of the catalystsystems in accordance with the invention. Inert carrier materialscomprising oxygen compounds of aluminum, titanium, and particularlysilicon, or mixtures of various such oxygen compounds, or siliconcarbide, are preferred.

The ratio of the catalytically active substance, that is of theheteropolymolybdate mixture of the general formula

    H.sub.a M.sub.b P.sub.c Mo.sub.d V.sub.e O.sub.f,

to the carrier may vary within certain limits. In general, thecatalytically active substance will be from 5 to 80 percent, andpreferably from 15 to 70 percent, of the total weight including thecarrier. For use as a catalyst system, the product so obtained may beput into a proper size and shape, for example by granulation,pelletizing, or tableting. The size and shape of the catalysts to beused depend mainly on the reaction technique employed in theoxydehydrogation, for example reaction in a fixed bed reactor. Shapingmay be followed by annealing of the catalyst system for from about 1hour to about 24 hours, for example at temperatures ranging from 200° C.to 400° C.

The catalyst system in accordance with the present invention isdistinguished by good activity and selectivity and at the same time bytechnically appropriate long term behavior in the oxidativedehydrogenation of isobutric acid or its esters, the latter beingprimarily the lower esters having from one to three carbon atoms in thealcohol group of the ester, and particularly isobutyric acid methylester. The conversion is effected at temperatures ranging from about250° C. to about 400° C. and preferably between 300° C. and 380° C. Thedwell time generally ranges from 0.1 to 5 seconds.

The oxydehydrogenation method in accordance with the present inventionis a gas phase reaction with oxygen as the oxidizing agent, for examplein the form of air. In addition to the isobutyric acid or ester andoxygen reagents, the gas mixture used in particular contains nitrogenand, optionally, also water in the form of steam. The molar ratio of thecomponents of the gas mixture, in the order indicated, advantageouslyranges from 1:(1-4):(4-20):(0-5) moles, the preferred molar ratio being1:(1-2):(4-10):(0.5-2).

Experience has shown that the presence of water has a favorable effecton the reaction. The inventive mixed heteropolymolybdate catalysts canbe used in the reactors commonly employed, for example using pressure.Suitable pressures range from 0.1 to 5 bar and are preferably from 0.5to 2.5 bar. The catalysts should be adapted with respect to size andshape to the reaction technique to be used.

A better understanding of the present invention and of its manyadvantages will be had from the following Examples, given by way ofillustration.

EXAMPLES (A) Preparation of mixed catalysts Example 1 H₃.5 PMo₁₁.5 V₀.5O₄₀

0.063 mole of H₄ PMo₁₁ VO₄₀ is introduced into a reactor as a 10 weightpercent aqueous solution and 0.063 mole of H₃ PMo₁₂ O₄₀ is then added.

The mixture is heated to reflux for 30 minutes at 100° C. The clearorange colored solution is mixed with 80.5 g of diatomaceous earth(kieselguhr) and 16.1 g of powdered silica gel and evaporated withstirring to leave a paste. The latter is oven dried for 1 hour at 110°C. and for 3 hours at 200° C. The catalyst is then calcined in a mufflefurnace for 3 hours at 300° C. and crushed into pieces about 5 mm insize.

Example 2 Cu₀.2 H₃.1 PMo₁₁.5 V₀.5 O₄₀

0.063 mole of H₄ PMo₁₁ VO₄₀ is introduced into a reactor as a 10 weightpercent aqueous solution and 0.063 mole of H₃ PMo₁₂ O₄₀ and 0.0252 moleof CuO are added.

After brief boiling (for 5 minutes), 81.4 g of diatomaceous earth and16.3 g of powdered silica gel are added. Further processing is carriedout as in Example 1.

Example 3 Cs₀.2 H₃.3 PMo₁₁.5 V₀.5 O₄₀

0.063 mole of H₄ PMo₁₁ VO₄₀ is used as a 10 weight percent aqueoussolution and 0.063 mole of H₃ PMo₁₂ O₄₀ and 0.0126 mole of Cs₂ CO₃ areadded.

The mixture is mixed with 81.4 g of diatomaceous earth and 16.3 g ofpowdered silica gel, evaporated with stirring to leave a paste, andfurther processed as described in Example 1.

Preparation of comparative catalysts Comparative Example 1 H₄ PMo₁₁ VO₄₀

The heteropolyacid was prepared as described in U.S. Pat. No. 4,146,574.The preparation of a 70 percent catalyst was carried out as described inExample 1.

Comparative Example 2 Cu₀.2 H₃.6 PMo₁₁ VO₄₀

The preparation of the 70 percent Cu₀.2 H₃.6 PMo₁₁ VO₄₀ catalyst wascarried out as described in Example 2.

Comparative Example 3 Cs₀.1 H₃.9 PMo₁₁ VO₄₀

0.063 mole of H₄ PMo₁₁ V₁ O₄₀ is used as a 10 weight percent solutionand 0.0032 mole of Cs₂ CO₃ is added. The further preparation of a 70percent catalyst is carried out as described in Example 3.

Example 4 Cu₀.05 Cs₀.1 H₃.3 PMo₁₁.5 V₀.5 O₄₀

0.063 mole of Cu₀.1 H₃.8 PMo₁₁ VO₄₀ is used as a 10 weight percentaqueous solution and 0.063 mole of H₃ PMo₁₂ O₄₀ and 0.0063 mole of Cs₂CO₃ are added.

The mixture is heated in a 3-liter beaker for 30 minutes, mixed with81.4 g of diatomaceous earth and 16.3 g of powdered silica gel, andprocessed further as described in the other Examples.

(B) Oxydehydrogenation of isobutyric acid Examples 1 to 4 andcomparative Examples 1 to 3 General testing procedure

A vaporous mixture of isobutyric acid and oxygen (as air) in a molarratio of 1:1:5 is reacted over the 70 percent catalysts (see followingTable I) in a circulating reactor known in the art (cf. published GermanPatent Application DOS 30 19 731) at 320° C. and with a dwell time of0.6 second. The catalyst loading is 2.5 kg of isobutyric acid per kg ofcatalytic mass per hour. The reaction gas is then analyzed by gaschromatography.

                                      TABLE I                                     __________________________________________________________________________    CATALYST                     CONVERSION (%)*                                                                          SELECTIVITY (%)*                      as prepared in                                                                             Catalytically effective substance                                                             C.sub.1                                                                          C.sub.250                                                                         C.sub.500                                                                         S.sub.1                                                                          S.sub.250                                                                         S.sub.500                      __________________________________________________________________________    Example 1    H.sub.3.5 PMo.sub.11.5 V.sub.0.5 O.sub.40                                                     71 73  71  50 53  55                             Comparative  H.sub.4 PMo.sub.11 V.sub.1 O.sub.40                                                           75 71  64  60 60  60                             example 1                                                                     Example 2    Cu.sub.0.2 H.sub.3.1 PMo.sub.11.5 V.sub.0.5 O.sub.40                                          64 57  47  72 66  58                             Comparative  Cu.sub.0.2 H.sub.3.6 PMo.sub.11 V.sub.1 O.sub.40                                              50 20  --  72 61  --                             example 2                                                                     Example 3    Cs.sub.0.2 H.sub.3.3 PMo.sub.11.5 V.sub.0.5 O.sub.40                                          65 58  52  55 62  62                             Comparative  Cs.sub.0.1 H.sub.3.9 PMo.sub.11 V.sub.1 O.sub.40                                              69 48  39  63 66  63                             example 3                                                                     Example 4    Cu.sub.0.05 Cs.sub.0.1 H.sub.3.3 PMo.sub.11.5 V.sub.0.5                       O.sub.40        64 61  53  70 71  68                             No Comparative example                                                        __________________________________________________________________________     *After the number of hours indicated by the subscripts.                  

Example 5 Cr₀.2 H₃.3 PMo₁₀.5 V₁.5 O₄₀

A 10 percent by weight solution of 0.063 mole of H₄ PMo₁₁ VO₄₀ and a 10percent solution of 0.063 mole of H₅ PMo₁₀ V₂ O₄₀ are combined at roomtemperature and then further mixed with a 10 percent solution of 0.025mole of CrO₃.

The resulting solution is combined with 80.5 g of kieselguhr and 16.1 gof powdered silica gel and further worked up as in Example 1.

Example 6 Fe₀.2 Bi₀.2 H₂.3 PMo₁₁.5 V₀.5 O₄₀

0.063 mole of H₄ PMo₁₁ VO₄₀ is prepared as a 10 percent aqueous solutionand 0.063 mole of H₃ PMo₁₂ O₄₀ is added. 0.025 mole of iron(III)nitrate.sup.. H₂ O and 0.025 mole of bismuth(III)nitrate .sup.. 5H₂ O aredissolved in about 15 ml of HNO₃ and are added to the solution of H₃.5PMo₁₁.5 V₀.5 O₄₀.

The solution is combined with 80.5 g of kieselguhr and 16.1 g ofpowdered silica gel and further worked up as in Example 1.

Examples 7-10

The preparation of these catalysts follows the procedures given inExample 5. For dosing with metal according to the formulas given infollowing Table II, Ce was added as Ce(NO₃)₄, Mn as Mn(CO₃)₂, Ti asTi(O-i-propyl)₄, Cu as CuO, and Cs as Cs₂ CO₃.

Example 11 Cu₀.2 H₂.8 PMo₁₁.8 V₀.2 O₄₀

0.1 mole of H₃ PMo₁₂ O₄₀ is dissolved together with 0.025 mole of CuO in500 ml of H₂ O with stirring at 100° C. and subsequently combined with0.025 mole of H₄ PMo₁₁ VO₄₀ (20 percent solution by weight).

The clear solution is combined with 82.2 g of kieselguhr and 16.4 g ofpowdered silica gel and further worked up as in Example 1.

Example 12 Cu₀.2 As₀.1 H₂.3 PMo₁₁.8 V₀.2 O₄₀

0.1 mole of H₃ PMo₁₂ O₄₀ is dissolved together with 0.0063 mole of As₂O₅ and 0.025 mole of CuO in 500 ml of H₂ O with stirring at 100° C. andsubsequently combined with 0.025 mole of H₄ PMo₁₁ VO₄₀ (20 percentsolution by weight). The clear solution is combined with 82.2 g ofkieselguhr and 16.4 g of powdered silica gel and further worked up as inExample 1.

Example 13 H₄.6 PMo₁₀.4 V₁.6 O₄₀

1.3 mole of MoO₃ are heated for 24 hours at 100° C. together with 0.1mole of V₂ O₅ and 0.125 mole of H₃ P₄ (85.9 percent) in 2000 g ofdistilled water.

The solution is worked up as in Example 1.

Example 14 Rb₀.2 H₃.3 PMo₁₁.5 V₀.5 O₄₀

0.063 mole of H₃ PMo₁₂ O₄₀ and 0.025 mole of RbNO₃ are added to a 10percent aqueous solution of 0.063 mole of H₄ PMo₁₁ VO₄₀.

The mixture is combined with 80.5 g of diatomaceous earth and 16.1 g ofpowdered silica gel and worked up as in Example 1.

Example 15 K₀.1 H₄.4 PMo₁₀.5 V₁.5 O₄₀

The catalyst is prepared as in Example 5. K is added as KNO₃ in anamount required by the formula.

Preparation of the Comparative Catalysts

The catalysts for comparative Examples 5-9, 11, and 13 are prepared by aprocedure analogous to that used in the Examples to which they pertainby combination of the relevant metal compound with H₅ PMo₁₀ V₂ O₄₀ or H₄PMo₁₁ VO₄₀.

B. Oxydehydrogenation of Isobutyric Acid

The catalysts of Examples 5-15 and comparative Examples 5-9, 11 and 13were used for the oxydehydration of isobutyric acid using the followinggeneral test procedure:

A vaporous mixture of isobutyric acid, oxygen (from air), and nitrogenin a mole ratio of 1:1.5:7.71 is reacted by passing it over the 70percent catalyst (cf. Table II) in a circulating reactor at 340° C. witha dwell time of 1 second. The loading of the catalyst is 1.25 kg ofisobutyric acid/kg of catalytic mass/hr. The reaction gas issubsequently analyzed by gas chromatography.

Conversion and selectivity as a function of time are reported in TableII.

                                      TABLE II                                    __________________________________________________________________________    Catalyst as  Catalytically effective                                          prepared in  substance      Conversion* (%)                                                                        Selectivity* (%)                         __________________________________________________________________________    Example 5    Cr.sub.0.2 H.sub.3.3 PMo.sub.10.5 V.sub.1.5 O.sub.40                                         C.sub.0                                                                          C.sub.4.8                                                                        C.sub.72                                                                         S.sub.0                                                                          S.sub.48                                                                         S.sub.72                                                       74.3                                                                             71.5                                                                             69.2                                                                             67.5                                                                             70.8                                                                             71.2                               Comparative  Cr.sub.0.2 H.sub.3.8 PMo.sub.10 V.sub.2 O.sub.40                                             C.sub.0                                                                          C.sub.51                                                                         C.sub.74                                                                         S.sub.0                                                                          S.sub.51                                                                         S.sub.74                           Example 5                   61.4                                                                             58.5                                                                             53.9                                                                             66.0                                                                             67.0                                                                             64.4                               Example 6    Fe.sub.0.2 Bi.sub.0.2 H.sub.2.3 PMo.sub.11.5 V.sub.0.5                        O.sub.40       C.sub.0                                                                          C.sub.60                                                                         C.sub.100                                                                        S.sub.0                                                                          S.sub.60                                                                         S.sub.100                                                      82.5                                                                             83.8                                                                             82.8                                                                             61.8                                                                             61.1                                                                             60.5                               Comparative  Fe.sub.0.2 Bi.sub.0.2 H.sub.2.8 PMo.sub.11 VO.sub.40                                         C.sub.0                                                                          C.sub.40                                                                         C.sub.86                                                                         S.sub.0                                                                          S.sub.40                                                                         S.sub.86                           Example 6                   82.5                                                                             79.6                                                                             77.4                                                                             67.0                                                                             70.5                                                                             69.0                               Example 7    Ce.sub.0.2 H.sub.3.7 PMo.sub.10.5 V.sub.1.5 O.sub.40                                         C.sub.0                                                                          C.sub.55                                                                         C.sub.128                                                                        S.sub.0                                                                          S.sub.55                                                                         S.sub.128                                                      86.5                                                                             86.8                                                                             86.7                                                                             71.0                                                                             70.9                                                                             71.7                               Comparative  Ce.sub.0.2 H.sub.4.2 PMo.sub.10 V.sub.2 O.sub.40                                             C.sub.30                                                                         C.sub.80                                                                         C.sub.124                                                                        S.sub.30                                                                         S.sub.79                                                                         S.sub.124                          Example 7                   74.1                                                                             72.8                                                                             70.9                                                                             72.5                                                                             70.6                                                                             76.0                               Example 8    Mn.sub.0.2 H.sub.4.1 PMo.sub.10.5 V.sub.1.5 O.sub.40                                         C.sub.0                                                                          C.sub.63                                                                         C.sub.71                                                                         S.sub.0                                                                          S.sub.63                                                                         S.sub.71                                                       82.8                                                                             80.5                                                                             79.7                                                                             66.4                                                                             68.9                                                                             71.9                               Comparative  Mn.sub.0.2 H.sub.4.6 PMo.sub.10 V.sub.2 O.sub.40                                             C.sub.18                                                                         C.sub.72                                                                         C.sub.94                                                                         S.sub.18                                                                         S.sub.71                                                                         S.sub.94                           Example 8                   71.0                                                                             62.0                                                                             59.9                                                                             75.7                                                                             70.7                                                                             64.0                               Example 9    Ti.sub.0.2 H.sub.3.7 PMo.sub.10.5 V.sub.1.5 O.sub.40                                         C.sub.0                                                                          C.sub.47                                                                         C.sub.73                                                                         S.sub.0                                                                          S.sub.47                                                                         S.sub.73                                                       77.4                                                                             79.2                                                                             78.4                                                                             72.5                                                                             74.4                                                                             74.5                               Comparative  Ti.sub.0.2 H.sub.4.2 PMo.sub.10 V.sub.2 O.sub.40                                             C.sub.0                                                                          C.sub.48                                                                         C.sub.73                                                                         S.sub.0                                                                          S.sub.48                                                                         S.sub.73                           Example 9                   70.9                                                                             68.3                                                                             66.6                                                                             74.1                                                                             75.4                                                                             72.8                               Example 10   Cu.sub.0.1 Cs.sub.0.1 H.sub.4.2 PMo.sub.10.5 V.sub.1.5                        O.sub.40       C.sub.0                                                                          C.sub.46                                                                         C.sub.73                                                                         S.sub.0                                                                          S.sub.46                                                                         S.sub.73                                                       84.1                                                                             83.3                                                                             83.8                                                                             75.3                                                                             76.2                                                                             76.0                               No Comparative Example                                                        Example 11   Cu.sub.0.2 H.sub.2.8 PMo.sub.11.8 V.sub.0.2 O.sub.40                                         C.sub.0                                                                          C.sub.56                                                                         C.sub.75                                                                         S.sub.0                                                                          S.sub.56                                                                         S.sub.75                                                       90.6                                                                             89.3                                                                             88.8                                                                             55.6                                                                             56.3                                                                             56.8                               Comparative  Cu.sub.0.2 H.sub.3.6 PMo.sub.11 VO.sub.40                                                    C.sub.0                                                                          C.sub.48                                                                         C.sub.95                                                                         S.sub.0                                                                          S.sub.48                                                                         S.sub.95                           Example 11                  81.5                                                                             79.8                                                                             76.5                                                                             72.9                                                                             77.3                                                                             77.7                               Example 12   Cu.sub.0.2 As.sub.0.1 H.sub.2.3 PMo.sub.11.8 V.sub.0.2                        O.sub.40       C.sub.0                                                                          C.sub.50                                                                         C.sub.72                                                                         S.sub.0                                                                          S.sub.50                                                                         S.sub.72                                                       85.5                                                                             84.5                                                                             84.8                                                                             62.8                                                                             65.1                                                                             63.7                               No Comparative Example                                                        Example 13   H.sub.4.6 PMo.sub.10 V.sub.1.6 O.sub.40                                                      C.sub.42                                                                         C.sub.66                                                                         C.sub.90                                                                         S.sub.42                                                                         S.sub.65                                                                         S.sub.90                                                       85.5                                                                             85.9                                                                             86.3                                                                             71.3                                                                             72.9                                                                             73.1                               Comparative  H.sub.5 PMo.sub.10 V.sub.2 O.sub.40                                                          C.sub.0                                                                          C.sub.26                                                                         C.sub.54                                                                         S.sub.0                                                                          S.sub.26                                                                         S.sub.54                           Example 13                  80.4                                                                             80.1                                                                             80.4                                                                             68.0                                                                             68.4                                                                             68.4                               Example 14   Rb.sub.0.2 H.sub.3.3 PMo.sub.11.5 V.sub.0.5 O.sub.40                                         C.sub.21                                                                         C.sub.43                                                                         C.sub.117                                                                        S.sub.21                                                                         S.sub.43                                                                         S.sub.117                                                      84.1                                                                             84.0                                                                             84.0                                                                             63.5                                                                             63.7                                                                             64.0                               **                                                                            Example 15   K.sub.0.1 H.sub.4.4 PMo.sub.10.5 V.sub.1.5 O.sub.4                                           C.sub.0                                                                          C.sub.50                                                                         C.sub.80                                                                         S.sub.0                                                                          S.sub. 50                                                                        S.sub.80                                                       81.0                                                                             80.8                                                                             80.8                                                                             69.5                                                                             70.5                                                                             70.8                               **                                                                            __________________________________________________________________________     *After the number of hours indicated by the subscripts.                       **No Comparative Example                                                 

What is claimed is:
 1. A composite heteropolymolybdate catalyst, adaptable to the oxydehydrogenation of isobutyric acid or a lower ester thereof to methacrylic acid or a lower ester thereof, respectively, said composite catalyst being a mixture having the formula, for the composite of

    H.sub.a M.sub.b P.sub.c Mo.sub.d V.sub.e O.sub.f,

wherein M is at least one member selected from the group consisting of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Zn, Cd, Al, Ce, Ti, Zr, Sn, Sb, Pb, Bi, Cr, Mn, Fe, Co, Ni, Cu, Ag, and As, a is greater than 0.1 but less than 6, b is 0 to 3, c is 0.5 to 1.5, d is greater than 9 but less than 12, e is 0.1 to 3, and f has a value determined by the valences and amounts of the elements other than oxygen, and consisting essentially of (A) a mixture of at least two members selected from the group consisting of(i) H_(3+x) PMo_(12-x) V_(x) O₄₀, wherein x is 1, 2, or 3 and (ii) metals salts thereof, the molar ratio of any member or combination of members to any other member in such a mixture being from 0.99:0.001 to 0.001:0.99, or consisting essentially of (B) a mixture of at least one member selected from the group consisting of said mixture (A)(i) and (A)(ii) and at least one member selected from the group consisting of(iii) H₃ PMo₁₂ O₄₀ and (iv) metal salts thereof, the molar ratio of said member (A)(i) and (A)(ii) to said member or members (B)(iii) and (B)(iv) being from 0.9:01 to 0.1:0.9.
 2. A catalyst as in claim 1 wherein b is 0.05 to 2.5.
 3. A catalyst as in claim 1, wherein M is Cu, Cs, Rb, K, Ti, Ce, Mn, Fe, Bi, Cr, or As.
 4. A catalyst as in claim 1 wherein M is at least one member selected from the group consisting of copper and cesium.
 5. A catalyst as in claim 1 present on an inert carrier.
 6. A method for the oxidative dehydrogenation of isobutyric acid or a lower ester thereof to methacrylic acid or a lower ester thereof, which method comprises contacting a gaseous mixture of said isobutyric acid or a lower ester thereof and oxygen in the presence of a catalyst as in claim
 1. 7. A method as in claim 6 wherein b is 0.05 to 2.5.
 8. A method as in claim 6 wherein M is at least one member selected from the group consisting of Cu, Cs, Rb, K, Ti, Ce, Mn, Fe, Bi, Cr, and As.
 9. A method as in claim 6 wherein M is at least one member selected from the group consisting of copper and cesium. 